So you made a mistake in your essay. Don't panic.

DON'T
--ask anyone around you for correction tape, liquid paper, etc. Your teacher has to assume you are cheating. (Which means a 0 on the essay...)
--waste time and ink scribbling out what you wrote in the first place.
--try to recopy the whole essay (no time!)

INSTEAD, DO:
--bring your own correction tape, etc.
--neatly put one line through the word(s) you want your teacher to ignore, and write the new stuff above it (that's why you double-spaced, remember?)
--insert large chunks of new material by putting an asterisk (*) where the new stuff should go, and then putting a matching asterisk at the end of your essay. Follow the second * with your new material.

Be sure your essay is legible (readable). The easiest way to do this is spend the planning time planning. The messiest essays are from people who are either too lazy or "too smart" to plan. (The groups of students who don't do planning are a) those failing the course and b) those getting 90% and above. When I ask them why they don't plan, group a) says, "I didn't have time," not realizing that planning actually SAVES them time. Group b) says, "I don't need to plan," and then hands in something completely unprofessional, full of scribbles and arrows--they OBVIOUSLY should have planned.)
Tidiness is not marked directly, but if I can't figure out what you want me to read first, second, etc, your "structure" mark is going to be pretty bad.